Composition and atomic order effects on the structural and magnetic transformations in ferromagnetic Ni-Co-Mn-Ga shape memory alloys

摘要

Within a certain range of Co content, ferromagnetic Ni-Co-Mn-Ga shape memory alloys undergo martensitic transformation (MT) between ferromagnetic austenite and paramagnetic martensite. This brings about an enlarged magnetization change with respect to the ternary Ni-Mn-Ga, approaching the requirements for magnetic field-induced MT. The MT temperatures, as well as the martensite and austenite Curie temperatures, depend on composition and on the atomic order degree. Therefore, for a given composition, thermal treatments can induce different sequences of magnetic and structural transitions and consequent variation of both magnetization and transformation entropy changes. Analysis of the structural and magnetic transitions undergone by a set of selected Ni_(50-x)Co_xMn_(25+y)Ga_(25-y) (x= 3-8, y = 5-7) alloys allows to build a phase diagram where the composition-temperature regions for different crystallographic and magnetic states of the alloys are shown. The temperature changes produced by water quench from 1070 K and subsequent aging at 520 K modify such phase diagram, restricting the conditions under which the above-mentioned alloys show enhanced magnetization jump. Further, the effects of composition and thermal treatment on the transformation entropy change are evaluated and correlated with the magnetic contribution.